Inert gases such as nitrogen, argon, helium, and the like are widely employed in industry to protect materials from exposure to oxidizing elements contained in ambient atmosphere. For example, inert gases such as argon, nitrogen, and helium are commonly used today to shield materials during welding, spraying metallic and ceramic materials by thermal and plasma techniques, depositing coatings by chemical vapor and physical vapor deposition techniques, and melting and refining ferrous and non-ferrous metals and alloys. They are also used to provide inert atmosphere for processing composites, semiconductor materials, and chemicals, packaging electronics and food products, removing dissolved gases from chemicals, fruit juices and edible oils, vulcanizing rubber and curing tires, and processing ferrous and non-ferrous metals and alloys, ceramics, composites, and metal matrix. Inert gases used in these applications are required to be substantially free of oxygen and moisture because the presence of these impurities results in oxidizing the processed materials.
Inert gases mixed with more than 0.1% hydrogen are widely employed by the Metals Processing Industry for heat treating operations such as annealing, hardening, brazing, and sintering ferrous and non-ferrous metals and alloys. The primary function of hydrogen gas is to prevent oxidation of metals and alloys during heat treatment as well as to maintain a reducing environment in the furnace . Inert gases used for heat treating metals and alloys are also required to be substantially free of moisture to prevent surface discoloration and/or oxidation during processing.
A major portion of dry, inert nitrogen used by the Metals Processing Industry for heat treating metals and alloys has been produced by distillation of air in large cryogenic plants. The cryogenically produced nitrogen is generally very pure (contains less than 10 ppm by volume PG,3 residual oxygen) and expensive. To reduce the cost of nitrogen, several non-cryogenic air separation techniques such as pressure swing adsorption and permeation have been recently developed and introduced in the market. The non-cryogenically produced nitrogen is much less expensive, but it contains residual oxygen (0.1 to 5% by volume) which makes a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen in processing oxygen sensitive materials very difficult.
Several techniques have been developed and used commercially today to purify and produce inert gases substantially free of residual oxygen and other impurities prior to using them in processing oxygen-sensitive materials. For example, Cu/CuO and Ni/NiO based catalysts have been used extensively to purify inert gases by chemically scavenging residual oxygen from them. These catalyst systems are described in detail in BASF Technical Leaflet on BASF-Catalyst R3-11 and U.S. Pat. No. 4,713,224. Since the oxygen absorption capacity of these catalyst systems is limited, they are generally used to purify inert gases containing less than 1,000 ppm or 0.1% by volume oxygen impurity. They are, therefore, not economically attractive for removing residual oxygen from nitrogen streams containing more than 0.1% oxygen impurity. Furthermore, they are not suitable for producing nitrogen-hydrogen atmospheres required for many heat treating applications.
Inert gases containing 0.1% or more of oxygen as an impurity have been purified by converting oxygen to moisture by reaction with hydrogen over platinum group metal catalysts. Such techniques have been disclosed in U.S. Pat. Nos. 5,004,489 and 4,960,579 and in Australian Patent Applications AU 45561/89 and AU 45562/89. The inert gas streams produced by such techniques are contaminated with moisture. They are, therefore, not suitable for producing moisture-free, nitrogen-hydrogen atmospheres required for many heat treating applications.
U.S. Pat. No. 3,535,074 discloses a process for purifying inert gases containing oxygen as an impurity. According to patentees, a pure inert gas stream is produced by converting a part of the oxygen contained in the gas stream to moisture by mixing with hydrogen over a platinum group metal catalyst followed by removing hydrogen, moisture and, remaining oxygen by using a copper or nickel catalyst. The disclosed process, therefore, is not suitable for producing nitrogen-hydrogen atmospheres required for many heat treating applications.
U.S. Pat. Nos. 4,931,070, 5,004,482, 5,077,029, and 5,122,355 and European Patent Application 91109189.0 disclose various processes for producing high purity nitrogen from non-cryogenically generated nitrogen stream. The non-cryogenically generated nitrogen stream containing residual oxygen is purified by mixing it with a controlled amount of hydrogen, converting residual oxygen to moisture over a platinum group metal catalyst, and removing moisture by using a dryer. The amount of hydrogen used in these processes is controlled very precisely to produce high purity nitrogen containing less than 0.1% or 1,000 ppm hydrogen impurity. These processes do not, therefore, disclose producing nitrogen-hydrogen atmospheres with more than 0.10% hydrogen required for many heat treating applications.
French Patent Applications (Publication numbers 2,639,249 and 2,639,251) disclose processes for producing nitrogen-hydrogen atmospheres from non-cryogenically generated nitrogen. In these processes non-cryogenically generated nitrogen containing residual oxygen is mixed with hydrogen and reacted over a platinum group metal catalyst to convert residual oxygen to moisture. The moisture is subsequently removed using a refrigeration dryer, producing nitrogen-hydrogen stream containing more than 350 ppm moisture. The processes disclosed in these patent applications are, therefore, not suitable for producing substantially moisture-free, nitrogen-hydrogen atmospheres required for many heat treating applications.